Since the industrial revolution, modern society has been built by consuming a large amount of fossil fuels, however, carbon dioxide concentration in the atmosphere has increased, and further, this increase has been more accelerated by environmental destruction such as deforestation, etc. Global warming is caused by an increase of greenhouse gases such as carbon dioxide, Freon, and methane in the atmosphere, such that it is particular important to reduce the atmospheric concentration of carbon dioxide highly contributing to global warming, and various studies into emission regulation, immobilization, etc., have been conducted on a global scale.
Among the studies, copolymerization of carbon dioxide and epoxide developed by Inoue, et al., is expected as a reaction for solving the problems of global warming, and has been actively researched in view of chemical immobilization of carbon dioxide and in view of the use of carbon dioxide as a carbon resource. Particularly, a polyalkylene carbonate resin obtained by the polymerization of carbon dioxide and epoxide has recently received particular attention as a kind of biodegradable resin.
Various catalysts for preparing the polyalkylene carbonate resin have been studied and suggested for a long time, and as representative examples thereof, zinc dicarboxylate-based catalysts such as a zinc glutarate catalyst, etc., in which zinc and dicarboxylic acid are bonded to each other have been known.
The zinc dicarboxylate-based catalyst, and as a representative example, a zinc glutarate catalyst, is formed by reacting a zinc precursor with a dicarboxylic acid such as a glutaric acid, etc., and has a shape of fine crystalline particles. However, the zinc dicarboxylate-based catalyst having the crystalline particle shape has a difficulty in being controlled to have a uniform and fine particle size in a preparation process thereof. The existing zinc dicarboxylate-based catalyst has a particle size of a nanometer scale, but an aggregate having an increased particle size and a decreased surface area is formed in a medium by aggregation of catalyst particles such that at the time of preparing the polyalkylene carbonate resin, the activity may be deteriorated.
In this regard, it has been found that as the zinc precursor used in the preparation of the zinc dicarboxylate-based catalyst has a smaller size, it has an important influence on the activity increase of the produced catalyst.
Generally, zinc oxide powder used as a zinc source (zinc precursor) in the preparation of the zinc dicarboxylate-based catalyst is an ionically bonded substance having a particle size of several tens to several hundred nanometers and a specific surface area of about 10 m2/g. This zinc oxide powder is highly polar, and thus is dispersed well in polar solvents, but particles are aggregated with each other in non-polar solvents to form very large aggregates. When a catalytic synthetic reaction is carried out under the aggregation of particles, a heterogeneous reaction occurs, and therefore heterogeneity in crystallinity of the resulting catalyst is increased, thus decreasing the activity of the catalyst, in view of the entire reaction system.
In this regard, International Publication WO 2013/034489 and U.S. Pat. No. 7,405,265 disclose a method for increasing a specific surface area of a zinc dicarboxylate-based catalyst by using an emulsifier or a non-ionic surfactant as a templating agent in a polar solution. However, the results of the previous literatures or studies showed that when zinc oxide and glutaric acid are used as raw materials for the synthesis of zinc dicarboxylate-based catalyst, a catalyst synthesized in a non-polar solvent generally has higher activity than a catalyst synthesized in a polar solvent. Thus, there is still a limitation in the improvement of the catalytic activity by the method of using the polar solvent.
In addition, the activity of the existing zinc dicarboxylate-based catalyst itself is not satisfactory. Moreover, in the case of the zinc dicarboxylate-based catalyst, it is not easy to disperse and control the catalyst particles in a reaction solution due to the non-uniformity of the particle diameter.